All the forces and quantities of matter can be divided into their individual parts. For example the photon (light), the phonon (sound), the proton (matter), and electricity (electron) are all example of individual units. J.J. Thompson was born in 1856 and lived until 1940. Thompsonn was not only an English pysicist, but also a Nobel laureate who created the mass spectrometer and discovered the electron and the existence of isotopes. However, Thompson is heavily associated with his use of the Cathode Ray Tube experiment to discover the electron. A cathode ray tube is a vacuum tube with electrodes on each side. An electrical current is sent through the vacuum tube creating a "ray" that can be deflected by both a magnet and and electrical field. Thompson realized that the experiment could not be conducted because the tube contained traces of gas left by the glass-blower. Thompson then heated the tubes, which allowed the traces of gas to leave the tube. Once the vacuum was achieved, Thompson's used a magnet/electrical field to study the deflection of the "ray." From this, Thompson concluded that the "ray" was composed of a negatively charged particle, the electron, and the experiment was successful. Below is an animation of a cathode ray tube:
After the electron was discovered, Millikan would be the physicist to measure the charge of an electron using oil, an iron pot, and electrical power. However, during this time period there was a strong influence from Europe, regarding advances in the sciences. These advances fueled Millikan's ambition and included: Nobel Prize winner and Univeristy of Chicago professor Albert Michelson's creation of the instrument to measure distances by the interference of light waves; William Ramsay's discovery of the elements helium, xenon, krypton, and neon; William Rontgen's discovery of x-rays; Guglielmo Marconi's adaption of the wireless telegraph; Marie Curie nobel prizes form her work in physics and chemistry; Albert Einstein's Theory of Relativity; work by Max Planck and Johannes Stark; and J.J Thompson and A.J. Wilson's discovery of the effects of x-ray radiation and the cloud chamber method for measuring charge. With all these advances at the frontier of physics, Millikan was determined to use his skills with precise measurments in order to push the frontier slightly farther.
Therefore, Millikan performed the famous Oil-Drop experiment. Millikan first ionized a gaseous cloud, and utilized both x-rays and radium. Millikan then used a powerful electrical field to balance the force of gravity and suspend the cloud of water without motion. However, this presented a problem because the water cloud would evaporate. Therefore, Millikan used oil droplets that would not disappear in order to detect the effect of an electron. Millikan theorized that if an electrical field is applied to an electrically charged oil falling through the air, then the resulting charge of the electron could be determined using F=ma:
ma = summation (F)
F(gravity) = mg and viscosity
F(viscosity) = 6(pi)Rnv
where R = the radius of the sphere, n = the viscosity of the air, and v = velocity.
F(viscosity) is a constant speed, however the velocity grows until viscosity balances gravity.
v = mg/ 6(pi)Rn
Below is a video showing the principle of viscosity:
Millikan then needed to measure the speed in order to find out how big each drop of oil was because they were to small to see with the human eye:
D(density) = m/(4/3)(pi)R^3 mg/6(pi)Rn = v
R^2D = m/(4/3)(pi)R m/(pi)R = 6nv/g
(4/3)R^2D = m/(pi)R
(4/3)R^2D = 6nv/g
R = sqrt (6nv/(4/3)gp)
The electric field creates an electric force so F(electricity) = electric fields strength times the strength on the oil drop. F(e) = qe because the force of electricty will be an integer multiple of the fundamental unit of electricity.. The electric field will drive the drop up until a constant speed is reached.
qE - mg = 6(pi)Rnv
v = qe - mg/6(pi)Rn
v (electric field on) = qe - mg/6(pi)Rn v (electric field off) = mg/6(pi)Rn
v(on) + v(off) = qe/6(pi)Rn
qe = (v(on) +v(off)) 6(pi)Rn
q = (v(on) +v(off)) 6(pi)Rn/e
Millikan took meticulous steps in order to ensure that his experiment yielded the correct result. Millikan minimized the turbulence of the droplets between the two plates by housing the plates in iron pots. The air was filtered through glass wool and an atomizer was used to spray the finest mist of oil droplets into the chamber. Light was filtered out by a solution of copper sulfate that coated a one meter tube of wate to eliminate the heat of water. Below is a video displaying actual footage of Millikan's experiment repeated:
Millikan found that the charge of an electron was 4.77 x 10^-10 or 1.602 x 10^-19 coulombs, a two percent difference from the prior experiment performed by Thompson. In 1923, Millikan was the first native born American to get the Nobel Peace Prize in physics.
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